1. Field of Invention
The present invention relates to a focus detection device for detecting the focus adjustment condition of a photographic lens. More particularly, the invention relates to a focus detector which accurately performs focus detection using a wide range of injected luminous flux from the photographic lens.
2. Description of Related Art
A prior art focus detector will now be explained using FIGS. 13 and 14. Referring to FIG. 13, a focus detector 20 provides a focus detecting optical system which comprises a condenser lens 21, a pair of diaphragm apertures 22a and 22b, a pair of separator lenses 23a and 23b and a pair of light receiving sensors 24a and 24b. The focus detector 20 is arranged near the predetermined image forming plane 31 of the photographic lens 30. The images transmitted through the condenser lens 21 and the pair of diaphragm apertures 22a and 22b become the entrance pupils 25 (see FIG. 14) of the focus detector 20.
FIG. 14 illustrates the entrance pupils 25 overlapping the injected luminous flux (light) 26 provided to the focus detecting optical system. As can be seen in FIG. 14, the prior art focus detector 20 only uses a small part of the injected luminous flux 26 for focus detection. In other words, the prior art focus detector 20 only detects the focusing position of a small part of the luminous flux within the injected luminous flux 26 of the photographic lens 30.
In general, injected luminous flux from a photographic lens suffers aberrations, such as, for example, a spherical aberration. Thus, one cannot properly select the focusing position of the entire luminous flux from the focusing position of the small part of the luminous flux within the injected luminous flux of the photographic lens. Accordingly, it is desirable to perform focus detection using as much of the luminous flux as possible. In order to do so, it is necessary to enlarge the diaphragm apertures 22a and 22b of the prior art focus detector 20. However, if the diaphragm apertures 22a and 22b are too large, the amount of shift of the images on the pair of light receiving sensors 24a and 24b is too great. Since the predetermined image forming plane 31 and the pair of light receiving sensors 24a and 24b share a common vertical plane, if the focusing of the photographic lens 30 is shifted in relation from the predetermined image forming plane 31, the image on the pair of light receiving sensors 24a and 24b will also shift. Therefore, the smaller the diaphragm apertures 22a and 22b are, the smaller the amount the image will shift, and conversely, the larger the diaphragm apertures 22a and 22b are, the larger the amount the image will shift. It should be noted that when the amount of the image shift is large, the contrast disappears, and the accuracy of the focus detection decreases.
In order to perform accurate focus detection using as wide a range of luminous flux as possible within the injected luminous flux of the photographic lens, a focus detector is known, such as, for example, the focus detector of Japanese Laid-Open Patent Application No. 7-333489, in which a light diffusion member is provided at, or near, a predetermined image forming surface of the photographic lens, and the entrance pupil areas of the focus detecting optical system are substantially made to be large. However, since this prior art focus detector also enlarges the entrance pupil area of the focus detecting optical system in the alignment direction of the pair of the entrance pupils, or in other words, the focus detecting direction, the focusing accuracy is adversely affected.